Photo-Alignment using Adsorbed Dichroic Molecules

A Planar Fresnel Lens in Reflection Type Based on Azo-Dye-Doped Cholesteric Liquid Crystals Fabricated by Photo-Alignment

This paper presents a focusing efficiency and focal length tunable planar Fresnel lens in reflection type based on azo-dye-doped cholesterol liquid crystal film. The Fresnel-like pattern of a pumping beam can be formed by a Sagnac interferometer. When the azo-dye molecules are irradiated by the pumping beam, the photoalignment effect will be induced in the bright (odd) zones due to the trans–cis photoisomerization of azo-dye molecules. Thus, the structures of cholesteric liquid crystals in the odd zones will reorient from the imperfectly planar textures to the perfectly planar textures. The different structures of cholesteric liquid crystals in two adjacent zones will give rise to phase difference for the reflected light and thus function as a Fresnel lens. The focusing efficiency of the proposed Fresnel lens can be controlled by the applied voltages and affected by the polarization state of incident light. Moreover, various focal lengths of the Fresnel lens can be achieved by rewriting a different center radius of the Fresnel-like pattern.

Exotic Property of Azobenzenesulfonic Photoalignment Material Based on Relative Humidity

Azobenzene photoalignment materials are highly effective for liquid crystal alignment with high sensitivity and rewritability. A strong relationship between relative humidity and the alignment quality of a thin layer of azobenzenesulfonic dye has been investigated, where the photoinduced phase retardation, order parameter, and anchoring strength of the alignment layer are influenced dramatically by relative humidity. Our results provide fabrication guidance for the photoalignment process in both display and photonic applications. In addition, an exotic substantial ordering enhancement is observed by increasing the relative humidity without further light illumination, where the self-assembly of the photoaligned material incorporated with water molecules is the underlying reason for the enhanced high ordering (S > 0.8). Based on X-ray diffraction and depolarized optical microscopy observation, together with the photoalignment quality, a semicrystalline structure of the humidified azobenzenesulfonic material is proposed. The transition from amorphous solid at low relative humidity to semicrystal at high relative humidity provides a new perspective of understanding the hydrophilic photoalignment materials.

Photoalignment of an azobenzene-based chromonic liquid crystal dispersed in triacetyl cellulose: single-layer alignment films with an exceptionally high order parameter

Single-layer thin alignment films of dye molecules are of growing importance, particularly for state-of-the-art LCD technology. Here we show that a sequential process involving the photoalignment and humidification of a chromonic liquid crystalline azobenzene (brilliant yellow; BY) dispersed in a triacetyl cellulose (TAC) matrix gives a thin alignment film with an exceptionally high order parameter (0.81). Spectroscopic and X-ray diffraction analyses of a BY/TAC composite film in each alignment process revealed that brief humidification triggers restructuring of the BY assembly from 1D nematic-like order to anisotropic 2D columnar order, resulting in the dramatic increase in the order parameter.